As the trend in product development becomes more miniaturized, the use of flexible substrates with high density also flourishes. To fulfill the demand of high density and fine pitch, the material used as the encapsulation film in flexible substrate has to possess better heat endurance capability, dimensional stability, electrical properties, and chemical resistance. The drilling techniques employed during processing of traditional flexible substrates, such as pre-punching or pre-drilling, can only achieve a minimal opening diameter of 800 μm, and other techniques like screen printing drilling can only achieve a minimum of 300 μm. On the other hand, though laser drilling can achieve a minimal opening diameter of 25 μm, its high production cost renders it uncompetitive. To solve this problem, the photosensitive encapsulation film materials are adopted, and by utilizing the lithography process, fine and precise patterns can be obtained. However, the photosensitive materials are mostly composed of epoxy resin and acrylic resin, both of which do not possess sufficient heat endurance capability and mechanical strength as a encapsulation film for applications in advanced products. Moreover, in regard to fulfilling the demand of halogen-free and phosphorus-free from the perspective of environmental protection, the UL-94V0 flame retardancy requirement to the epoxy resin and acrylic resin is a major obstacle. The photosensitive polyimide (PSPI) material has excellent heat stability and good mechanical, electrical, and chemical properties. PSPI can meet UL-94V0 flame retardancy requirement without the addition of flame retardants, and it does not need to be concerned with the issue of halogen-free and phosphorus-free, which makes PSPI an ideal material for use in the advanced flexible substrates with high density and fine pitch.
Traditional PSPI is usually consisted of polyamic acid and polyamide ester, both of which are its precursors and require a curing temperature as high as 350° C. to form polyimide after the lithography process. Such temperature gives rise to the problem of oxidation of copper circuits; and also the problem of excessive shrinkage to polyimide. Moreover, the thickness of encapsulation film produced this way is mostly less than 10 μm, which cannot satisfy the requirement of a thicker film of 20 μm or more.
Soluble PSPI materials are used as the encapsulation film for advanced flexible substrates, as can be shown in US2003/0176528, US2004/0247908, US2004/0265731, and US2004/0235992. Although they can be cured at a lower temperature of 230° C., the high percentage of photosensitizing particles (acrylic acid ester) contained therein also leads to the problem of worse flame retardancy. This problem necessitates the addition of flame retardants that contain phosphorus or halogen, which cannot meet the demand of halogen-free and phosphorus-free in the future.
Although soluble PSPI materials have a lower post-curing temperature, their solvent resistance are generally worse and require alkaline developing solution at high concentration to develop images, which reduces their applicability. In Reactive & Functional Polymers; 2003, 56, 59-73, JP2002341535 and JP2003345007; Masao Tomoi and colleagues have proposed a soluble polyimide having carboxyl groups on its backbone, and acrylic acid (ester) monomers having a tertiary amino group to react with the carboxyl groups to give rise to ionic bonds, thereby forming a negative PSPI material. Because the strength of ionic bonding is not as strong as covalent bonding, the PSPI materials having ionic bonding is suitable in the case where a thickness of PSPI film is 10 μm or less. If a thicker PSPI film (20 μm) is required, its exposure energy can reach as high as 8000 mj/cm2, which renders it unapplicable. Furthermore, the PSPI film resulted from post-curing has worse solvent resistance and alkaline resistance due to the presence of the aforementioned ionic bonding.